In the field of ionics, there is a trend to develop solid electrolytes as new ionic conductors to replace conventional electrolyte solutions and to develop applications for solid-state primary or secondary batteries and electric double layer capacitors. Conventional products, such as batteries with electrolyte solutions, have problems in long-term reliability, since leakage of the electrolyte solution out of the parts or elution of the electrode substance tends to occur. In contrast, products with solid electrolytes do not cause such problems and it is easy to decrease their thickness. Furthermore, solid electrolytes have excellent thermal resistance and are advantage ous in manufacturing products such as batteries.
Among batteries using a solid electrolyte, those using a polymer as a main component of the electrolyte have increased flexibility as compared with those using an inorganic substance, which allows the former to be processed into vari ous forms. However, such products as hitherto studied still have the problem that only a small amount of current can be taken out, since the solid polymer electrolyte has low ionic conductivity.
An example of such a solid polymer electrolyte is described in the British Polymer Journal (Br. Polym. J.), 7:319-327 (1975), which states that a compounded material consisting of a polyethylene oxide and a n inorganic alkali metal salt exhibits ionic conductivity as low as 10.sup.-7 S/cm at room temperature.
Recently, there have been many reports that a comb-shaped polymer having an oligooxyethylene in each side chain has improved ionic conductivity due to increased thermal motion of the oxyethylene chain, which contributes ionic conductivity. An example in which a polymethacrylic acid, having an oligooxyethylene added to its side chain, is compounded with an alkali metal salt is described in the Journal of Physical Chemistry (J. Phys. Chem.), Vol. 89, page 987 (1985). Another example where polyphosphazene, having an oligooxyethylene side chain, is compounded with an alkali metal salt is described in the Journal of American Chemical Society (J. Am. Chem. Soc.), Vol. 106, page 6854 (1984).
Recently, many studies have been made on lithium secondary batteries in which metal oxides or metal sulfides, such as LiCoO.sub.2, LiNiO.sub.2, LiMnO.sub.2 and MoS.sub.2, are used as positive electrodes. For example, batteries using positive electrodes made of MnO.sub.2 or NiO.sub.2 are reported in the Journal of Electrochemical Society (J. Electrochem. Soc.), Vol. 138 (No. 3), page 665 (1991). These batteries have drawn attention since they have high gravimetric or volumetric capacity.
Many reports have been made on batteries using an electroconductive polymer as an electroactive material. For example, a lithium secondary battery using polyanilines as the positive electrode has already been put on the market in the form of a coin-type battery for use as a backup source, by Bridgestone Co., Ltd. and Seiko Co., Ltd. as reported in, for example, "The 27th Symposium on Battery 3A05L and 3A06L" (1986). Polyaniline also attracts attention as an electroactive material for positive electrodes having high capacity and flexibility.
Lately, electric double layer capacitors which comprise polarizable electrodes made of a carbon material having a large specific area, such as activated carbon or carbon black, and an ionic conducting solution arranged between the electrodes, have been widely used as a memory backup source. For example, a capacitor having carbon-based polarizable electrodes and an organic electrolyte solution is described in "Kinou Zairyo", February 1989, page 33. An electric double layer capacitor using an aqueous sulfuric acid solution is described in "The 173rd Electrochemical Society Meeting Atlanta Georgia", May, 135(3):332 (1988). Also, a capacitor using highly electroconductive Rb.sub.2 Cu.sub.3 I.sub.3 Cl.sub.7 as an inorganic solid electrolyte is disclosed in Japanese Patent Application Laid-open No. 63-244570 (1988).
However, electric double layer capacitors using conventional electrolyte solutions have long-term use and reliability problems since leakage of the solution out of the capacitor tends to occur when used for a long time or when a high voltage is applied. On the other hand, electric double layer capacitors using conventional inorganic based ionic conducting substances have a problem in that the ionic conducting substance decomposes even at a low voltage and, hence, the output voltage is low.
The use of an ionic conducting substance using a polyphosphazene based polymer for a capacitor is disclosed in Japanese Patent Application Laid-open No. 4-253771 (1992). A capacitor using such an ionic conducting substance containing a polymer as the main component has an output voltage higher than the inorganic substance based ionic conducting substance, can be processed into various forms, and is easily sealed.
In this case, the ionic conductivity of the solid polymer electrolyte, which is as low as 10.sup.-4 to 10.sup.-6 S/cm, is still unsatisfactory and is defective in that the take-out current is small. Although it is possible to increase the ionic conductivity of the solid polymer electrolyte by the addition of a plasticizer, which gives fluidity to the electrolyte, it means that the electrolyte can no longer be treated as a complete solid. As a result, the film made of this polymer has poor film strength and the polymer has poor film formability and, hence, a short-circuit tends to occur when the polymer is used in an electric double layer capacitor or a battery and there is difficulty in sealing, as in the case of liquid ionic conducting substances. In addition, when it is assembled with a polarizable electrode, such as a carbon material, to produce a capacitor, it is difficult to homogeneously compound the solid polymer electrolyte with a carbon material having a large specific surface area, since both materials are solid.
The ionic conductivity of solid polymer electrolytes studied so far has improved up to about 10.sup.-4 to 10.sup.-5 S/cm, which is over two digits lower than that of liquid ionic conducting substances. Besides, at a temperature no higher than 0.degree. C., ionic conductivity drastically decreases to a lower level. Furthermore, when these solid electrolytes are incorporated in a device, such as an electric double layer capacitor, or incorporated in a battery in the form of a thin film, manufacturing problems arise since the solid electrolyte is hard to compound with the electrode and it is difficult to achieve a satisfactory contact between the electrolyte and the electrode.